Our work on CD97 began with the discovery of CD97 as a mitogen-induced gene in T cells. We were the first to demonstrate that the CD97 protein was post-translationally processed to a non-covalently associated long N-terminal extracellular region and a GPCR subunit. Subsequently, several additional members of the adhesion GPCR family were discovered, which have a similar processed structure, highly related GPCRs, and unique extracellular regions with the functionally related property of containing repeated adhesion motifs. One major challenge in investigating adhesion GPCRs has been an understanding of the signaling mechanisms for these receptors, in part because the identification of ligands has been difficult. Although CD97 was first identified in hematopoietic cells, a number of reports appeared describing elevated expression in various carcinomas, including prostate, thyroid, gastric, and colorectal cancers, with virtually no expression in the corresponding normal epithelia . To begin addressing a potential functional role for CD97 in tumorigenesis and/or progression, we investigated the effects of CD97 depletion or over-expression in preclinical models. In human prostate xenograft models, we determined that loss of CD97 resulted in decreased bone metastases and in decreased in vitro invasion to serum 45. We also established that increased CD97 expression correlates with clinical thyroid cancer progression. As an approach to dissecting the in vivo role of CD97 in de novo tumorigenesis, we investigated the effects of thyroid epithelium-specific CD97 over-expression upon progression in a mouse model of follicular thyroid cancer. We determined that CD97 expression led to increased ERK activation and Ki67 labeling in early neoplastic lesions and to accelerated thyroid carcinoma formation and vascular invasion . These preclinical models suggest a direct role of CD97 in cancer progression. Our understanding of CD97 signaling was greatly aided by two particularly informative observations. 1) As a result of producing and assaying recombinant CD97 extracellular domain, we determined that CD97 is a ligand for RGD-binding integrins that could stimulate alpha-v, beta-3 and alpha-v,beta-5 dependent endothelial cell migration . 2) By overexpressing the GPCR subregion of CD97, we demonstrated that CD97 signals via G alpha12/13 to RHO and ERK. We also observed unexpectedly that CD97 depleted prostate cells down-regulated pERK. This led us to consider whether a constituent in serum may be a ligand for CD97. By analyzing the major chemoattractants in serum, we observed that CD97 loss was particularly significant in inhibiting LPA-induced migration as well as reducing LPA signaling to RHOA and ERK. Using co-immunoprecipitation and in situ proximity ligation assays, we showed that CD97 and LPA receptor (also a GPCR) heterodimerize. These data suggested that CD97 amplifies signaling initiated through the LPA/LPA receptor axis. Because platelets highly express an RGD-binding integrin receptor (alpha-2b,beta-3), are the major source of LPA in serum, and promote metastasis by their interaction with tumor cells, we hypothesized that CD97 on tumor cells is a receptor for platelets that leads to LPA release and RHO-dependent signaling of tumor cells. This turned out to be the case. Tumor cells initiate platelet activation leading to the secretion of bioactive molecules, which promote metastasis. Inhibiting metastatic spread and reducing metastatic burden is an important therapeutic goal; however, treatment approaches directed toward platelets are limited by adverse side effects such as uncontrolled bleeding. Platelet receptors on tumors have not been well-characterized, resulting in a critical gap in knowledge concerning platelet-promoted metastasis. We identify a direct and necessary interaction between platelets and tumor cell CD97 that stimulates rapid bidirectional signaling. CD97 is an adhesion GPCR over-expressed in several cancers. Purified CD97 extracellular domain activated platelets. Platelet-derived LPA increased tumor invasiveness via CD97-LPAR heterodimer signaling. The amplification of platelet activation through a CD97-alpha-IIb, beta-3 dependent mechanism led to platelet granule ATP release and endothelial barrier retraction. Thus, tumor CD97 initiates platelet activation and acts to couple coincident tumor cell migration and vascular permeability to promote transendothelial migration. Consistent with this, CD97 was necessary for tumor cell induced vascular permeability in vivo and for metastasis formation in preclinical models. Importantly, germline loss of CD97 in mice inhibited endogenous, genetically-engineered tumor progression without adverse systemic effects. These findings support therapeutic targeting of tumor CD97 to prevent or ameliorate metastatic spread. We have generated an antibody directed against CD97 that inhibits platelet activation by plate bound recombinant CD97.